When a forum post is submitted, instead of immediately saving it to the database, send an e-mail to a reserved account.

The e-mail account is configured to use procmail and SpamAssassin to filter its mail

If the mail is flagged as spam, simply throw it away

If the mail is flagged as ham, post it back to the real submission URL, where it will be saved to the database for all users to see.

The reason I am considering SA rather than a dedicated forum spam filter is that I am familiar with it and pleased with its results. Some dedicated forum spam filters are clearly not as good (but feel free to mention a good one if you know it.)

Other than some features of SA not being applicable (which I can ignore) what problems am I likely to run into with this approach?

As I said not all functions will be useful, but some will. E.g. looking for the string "viagra" in the body of the message, auto-whitelisting of IPs, detecting attempts to use HTML tags to conceal part of the text (some bots don't check the target forum engine.)
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finnwJul 30 '09 at 10:23

What IPs? To SA, the IP will always look the same. It will be the IP of your webserver.
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innaMJul 30 '09 at 11:58

No, I would add the HTTP client IP address as a header.
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finnwJul 30 '09 at 13:27